Vaporization coating process and alloy therefor



p 15, 1959 c. J. SCOTT m-AL 2,904,451

VAPORIZATION COATING PROCESS AND ALLOY THEREFOR I Filed Dec. 5, 1957lnven tofs: Charles dScot=b Kenneth S. G.Per=twee y flzrf TheiT' A L-T'TWGH.

2,904,451 Patented Sept. 15, 1959 dice VAPORIZATION COATING PROCESS ANDALLOY THEREFOR Charles J. Scott, Cleveland Heights, and Kenneth S. G.Pertwee, Cleveland, Ohio, assignors to General Elee: tric Company, acorporation of New York Application December 5, 1957, Serial No. 700,799

6 Claims. (Cl. 117-97) Our invention relates to the formation ofcoatings by the vaporization of silver, and more particularly to a novelmethod and alloy for forming such coatings.

In the formation of metal coatings by thermal evaporation, it isdesirable to effect the vaporization of the metal by applying itdirectly to an electrically energized vaporizer or filament ofrefractory metal which is preferably located in an evacuated space. Thefilament must be of a metal of high melting point and low vapor pressureso that, for practical purposes, it should be a metal such asmolybdenum, tungsten, tantalum or columbium.

Some metals can be successfully vaporized from such filaments wherebythey melt and spread over the filament bp capillary attraction so thatsatisfactory evaporation then occurs. However, silver does not wet thefilament and therefore tends to ball up and fall from the filament indroplets instead of vaporizing. In order to overcome this disadvantage,it has been the practice to apply the silver to the filament in the formof an electroplated coating and the results have been generallysatisfactory. However, that method entails certain disadvantages. Forinstance, as applied to the formation of reflecting coatings on glasselectric lamp bulbs, each plated filament is used to coat a single bulbafter which it must be replated with silver before being used to coatanother bulb. It will be apparent that a considerable amount of laborand space is required to perform the plating operation; moreover, thespoilage of filaments is high due to continual handling.

In accordance with the present invention, we have found that theabove-mentioned disadvantages are overcome by placing the silver on thefilament or vaporizer in the form of an alloy of the silver with smallamounts of both silicon and lithium.

Small amounts of silicon alone added to the silver caused it to wet thevaporizer or reduced the surface tension of the silver and allowed it tospread out on the vaporizer, but it apparently also raised the meltingpoint. At any rate, after a few operations of the vaporizer, it requireda higher temperature to evaporate the alloy and as a result ofoverheating the vaporizer the reflecting coatings were sometimesdiscolored. Moreover, When silicon alone is alloyed with the silver itleaves a clinker type residue of a black material on the vaporizer aftera few uses thereof and subsequent loads of silver ball up in thevaporizer, so that it must then be replaced. Such replacement is adistinct disadvantage for automatic high-speed operation. The additionof lithium alone to silver apparently lowered its melting point and didnot leave a residue, but it did not always spread smoothly on thevaporizer and showed a tendency to ball up in the vaporizer.

However, it was found that by adding both lithium, which lowered thevaporizing temperature, and silicon, which wet the vaporizer, weobtained the advantages of both and the disadvantages of neither. Thelithium apparently acts as a scavenger and prevents the formation of theresidue left when silicon alone is added, so that the original vaporizerfilament can be used indefinitely and the process continued by merelysupplying another quantity of alloy for each bulb which is to be coated.

In general, it is desirable to maintain the amounts of both silicon andlithium at a minimum consistent with adequate wetting and avoidance ofthe aforesaid residue. We have found that best results are obtained witha composition by weight of 0.06% lithium, 0.04% silicon, balanceessentially silver. For high reflectivity, we prefer to employ silver ofhigh purity, for example 99.95% or better, depending upon the economicalaspect. The presence of copper is especially to be avoided since itresults in discoloration of the reflector coating.

In the coating of certain types of lamp bulbs, a portion of the silvercoating is removed by dissolving it with an acid such as a mixture ofchromic and sulphuric acids and water, and excessive amounts of lithiumwould tend to affect the Wetting of the surface by the acid, and it mayalso have adverse effect on the glass itself. Excessive amounts ofsilicon would tend to leave a residue on the filament and, in any event,the reflectivity of the silver would be adversely affected by excessiveamounts of any foreign material. 011 the other hand, the use of siliconalone, alloyed With the silver, results in the formation of the darkresidue on the filament, whereas the use of lithium alone is likewisenot satisfactory because of inadequate wetting of the vaporizer.

In those cases where a portion of the reflecting coating is to beremoved by the conventional practice of dissolving it with acid and thenWashing the bulb with Water, We have found that excessive amounts oflithium cause the coating to peel off when it is Washed, the lithiumap-' parently being dissolved by the water. In that case We prefer tolimit the lithium content to not more than about 0.06%, and satisfactoryresults may be obtained with a composition in the range of 0.04-0.08%silicon, 0.02- 0.06% lithium, balance silver. When the finished coatingis not to be Washed with Water, as by employing a mask over the portionof the bulb not to be coated, then satis factory results may be obtainedby compositions in the range of about 0.04-l.9% silicon, 0.02l.8%lithium, balance silver. However, in either case we prefer thecomposition 0.04% silicon, 0.06% lithium, balance silver.

For good results, it is necessary that the alloy be formed by meltingthe silver, silicon and lithium in the indicated amounts in a vacuum.The vacuum melting process removes gas from the silver and promotessmooth evaporation of the silver from the vaporizer filaments. The gasnormally present in silver may cause discoloration and bombardment orsputtering during vaporization from. the filament.

In preparing the alloy, the silver may be placed in a graphite cruciblein a vacuum furnace and melted by induction heating to a temperature ofabout 1200 C. The heating is continued until bubbling ceases, which maybe for about three or four minutes, and then the indi cated proportionsof silicon and lithium are dropped into the molten silver, the inductionheating is immediately terminated and the melt is poured into a mold.For ease of handling when adding the silicon and lithium, they may beWrapped in silver foil the weight of which is, of course, taken intoaccount. Alternatively, the lithium and silicon may be added to themolten silver in the form of separate silver-lithium and silver-siliconmaster alloys. The cast alloy is then preferably rolled and drawn toribbon form about 0.010 inch thick and about 0.110 inch Wide.

The accompanying drawing is an elevation, partly in section,illustrating one form of apparatus for carrying out the method inconnection with the coating of one type of electric lamp bulb.

Referring to the drawing, the glass bulb 1 is carried by a rubber gasket2 which may constitute part of one of the heads of a rotary vaporizingmachine. The gasket 2 is arranged to make a vacuum-tight seal with theneck portion 3 of the bulb and is provided with a conduit 4 forconnection to a source of vacuum. The gasket 2 also supports a pair ofconductors 5 which extend into the interior of the bulb 11 and whichsupport a helically coiled vaporizer filament 6 of one of the aforesaidrefractory metals preferably a stranded molybdenum wire. Other vaporizerforms may be employed, such as a basket of stranded molybdenum wire inthe form of a vertically disposed conical coil with its apex lowermost.A quantity of the silver-silicon-lithium alloy suflicient to coat theinterior of the bulb 1 tothe desired thickness is placed in thevaporizer coil 6, preferably in the form of a length of crimped ribbon7.

After the ribbon 7 has been placed in the vaporizer 6.and the bulb 1 hasbeen mounted in place on the gasket 2, the bulb is evacuated through theconduit 4 to a suitable pressure of 5 microns or less. The vaporizer 6isthen heated to a temperature above the melting point of the alloy,preferably about 1800" C., by application of'an electrical currentthrough the conductors 5. After the strip 7 has evaporated and depositedon the interior surface of the bulb 1 in the form of a reflectingmetallic film, the bulb is removed and the coating is subsequentlyremoved from the end face 8 thereof by dissolving it in a suitable acidin known manner. The vaporizer 6 is then supplied with another length ofribbon 7 for coating another bulb.

What we claim as new and desire to secure by Letters Patent of theUnited States is:

1. The method of coating articles by vaporizing silver from, a filamentof metal selected from the group consisting of molybdenum, tungsten,tantalum and columbium, wherein the silver is placed on the filament asan alloy consisting of 0.02-1.8% lithium, 0.041.9% silicon and. thebalance essentially silver, and is heated on said filament, and whereinthe silicon causes the silver to wet and adhere to the filament and thelithium serves to prevent formation on the filament of a dark residueotherwise left thereon due to the presence of the silicon, and whereinby continued application of heat the silver is vaporized and caused todeposit on the said article.

2. The method of coating articles by vaporizing silver from a filamentof metal selected from the group consisting of molybdenum, tungsten,tantalum and columbium, wherein the silver is placed on the filament asan alloy consisting of about 0.06% lithium, 0.04% silicon and thebalance essentially silver, and is heated on said filament, and whereinthe silicon causes the silver to wet and adhere to the filament and,the, lithium serves to prevent formation on the filament of a darkresidue otherwise left thereon due to the presence of the. silicon-,andwherein by continued application. of. heat; the. silver is vaporized andcaused to deposit on the saidiarticle.

3. The method of coating articles by vaporizing silver from a filamentof metal selected from the group consisting of molybdenum, tungsten,tantalum and columbium, wherein the silver is placed on the filament asan alloy consisting of 0.020.06% lithium, 0.04-0.08% silicon and thebalance essentially silver, and isheated on said filament, andwherein-the-silicon causes the. silver to wet and adhere to the filamentand the lithium serves,

to prevent formation on the filament of a dark residue otherwise leftthereon due to the presence of the silicon, and wherein by continuedapplication of heat the silver is vaporized and caused to deposit on thesaid, article, andsubsequently dissolving the silver off a, portion. of,the:

References Cited in the file of this patent UNITED STATES PATENTS1,863,645 Assmann June 21, 1932' 1,982,774 Winkler et al. Dec. 4, 19342,196,307 Hensel et al Apr. 9; 1940 2,221,285 Hensel et al Nov. 12, 19402,793,115 Bredzs et al May 21, 1957' 2,819,982 Westerveld et a1 J an.14, 1958

1. THE METHOD OF COATING ARTICLES BY VAPORIZING SILVER FROM A FILAMENTOF METAL SELECTED FROM THE GROUP CONSISTING OF MOLYBDENUM, TUNGSTEN,TANTALUM AND COLUMBIUM, WHEREIN THE SILVER IS PLACED ON THE FIALMENT ASAN ALLOY CONSISTING OF 0.02-1.8% LITHIUM, 0.01-1.9% SILICON AND THEBALANCE ESSENTIALLY SILVER, AND IS HEATED ON SAID FILAMENT, AND WHEREINTHE SILICON CAUSES THE SILVER TO WET AND ADHERE TO THE FILAMENT AND THELITHIUM SERVES TO PREVENT FORMATION ON THE FIALMENT OF A DARK RESIDUEOTHERWISE LEFT THEREON DUE TO THE PRESENCE OF THE SILICON, AND WHEREINBY CONTINUED APPLICATION OF HEAT THE SILVER IS VAPORIZED AND CAUSED TODEPOSIT ON THE SAID ARTICLE.